A conventional wiring board manufacturing method, in which wiring patterns are formed on both sides of a core substrate by a build-up process, is shown in FIGS. 12 and 13.
A process of producing the core substrate 22, in which the wiring patterns are formed on the both sides, is shown in FIG. 12. A base board 10, in which copper films are adhered on both sides, is shown in FIG. 12A. The base board 10 is formed by adhering copper films 11 on the both side faces of a board member 10a, which is made of epoxy resin including glass cloth. In FIG. 12B, through-holes 12 are bored in the base board 10 by a drill. In FIG. 12C, inner faces of the through-holes are plated (with copper) so as to electrically connect wiring patterns, which will be formed on the both sides of the base board 10. The copper layers 14 are plated on the inner faces of the through-holes.
In FIG. 12D, the through-holes 12 are filled with resin fillers 16. In FIG. 12E, the side faces of the base-board 10 are covered with copper by plating. With this structure, the both side faces of the base board 10 including end faces of the resin fillers 16 are entirely covered with copper layers 18. In FIG. 12F, wiring patterns 20 are formed by etching the copper layers 14 and 18 and the copper films 11 of the base board 10, so that the core substrate 22 can be produced.
A process of layering the wiring patterns on the both sides of the core substrate 22 is shown in FIG. 13. In FIG. 13A, wiring patterns 24 are formed on the both side faces of the core substrate 24 by the build-up process. Symbols 26 stand for insulating layers, and symbols 28 stand for vias for electrically connecting wiring patterns 24 in different layers. In FIG. 13B, photosensitive solder resists 30 are applied on surfaces of the build-up layers, and they are exposed and developed. In FIG. 13C, surfaces of the wiring patterns 24 are electroless-plated with nickel and gold, and exposed parts of the wiring patterns 24 are protected by plated layers 32. In FIG. 13D, solder bumps 34 are formed on electrodes of the wiring patterns 24, and the wiring patterns 24 are layered on the both sides of the core substrate 22, so that a wiring board 36 can be produced.
Since the base board 10 of the core substrate 36 is made of tough resin including glass cloth, the core substrate has high rigidity. However, in the conventional wiring board manufacturing method, in which the wiring patterns 24 are layered on the core substrate 22, which acts as a support, by the build-up process, distances between the through-holes 12 of the core substrate 22 cannot be shorter than a prescribed distance, so that wiring density cannot be high.
Electric characteristics of wiring boards can be improved by reducing thickness thereof, so thin wiring boards are required. However, if the core substrate is made thin, a special conveying mechanism is required, and rigidity of the wiring board is lowered so that the wiring board will be shrunk or waved due to stress caused in the steps of forming insulating layers, wiring patterns, etc. and wiring density cannot be high. Further, if the core substrate is made thin, a difference of thermal expansion coefficients between the wiring board and a semiconductor chip is made great, so that thermal stress therebetween must be great. To approximate the thermal expansion coefficient of the wiring board to that of the semiconductor chip, a metal core substrate, whose thermal expansion coefficient is low and nearly equal to that of the semiconductor chip, was used, but thermal stress between the core substrate and the build-up layers makes cracks in the build-up layers.
Thus, the present invention has invented to solve the above described problems, and an object of the present invention is to provide a wiring board manufacturing method, which is capable of forming wiring patterns with high density so as to correspond to highly integrated semiconductor chips, restraining thermal expansion of the wiring board, and increasing rigidity of the wiring board so as to restrain and resist thermal stress between the wiring board and semiconductor chips.